FIX: Check for Webpacker config file existence before trying to acces…

…s the configs

Fixes: #457

lib/mini_profiler_rails/railtie_methods.rb

@@ -52,7 +52,7 @@ def should_move?(child, node)
   end
 
   def get_webpacker_assets_path
-    if defined?(Webpacker)
+    if defined?(Webpacker) && Webpacker.config.config_path.exist?
       Webpacker.config.public_output_path.to_s.gsub(Webpacker.config.public_path.to_s, "")
     end
   end